An image forming apparatus such as a color copying machine or a printer includes an image scanning unit. The image scanning unit includes a casing as a scanner base, an exposure lamp provided below a transparent original placing table, an imaging optical system including a lens and a CCD sensor, and a mirror that reflects an image of an original to the imaging optical system.
In an example of the conventional image scanning unit, the casing includes a bottom wall, a pair of sidewalls, and a pair of end walls formed at both ends in a scanning direction. A lens base member is provided between the sidewalls. The imaging optical system is attached to the lens base member. The exposure lamp is mounted on a first carriage. The mirror is mounted on a second carriage. These carriages reciprocatingly move in the casing along carriage rails.
Both ends in a longitudinal direction of the carriage rails of the conventional image scanning unit are fixed to the end walls of the casing, respectively. Since the lens base member supports the imaging optical system in conjunction with the carriages, the lens base member is desirably fixed to the carriage rails without being directly fixed to the casing. The lens base member is fixed to an intermediate section in the longitudinal direction of the carriage rails. In this case, the weight of the lens base member and the imaging optical system is applied to the intermediate section in the longitudinal direction of the carriage rails. Therefore, if the carriages move along the carriage rails, a vibration mode may occur in the carriage rails. If such a vibration mode is transmitted to the imaging optical system, a quality of an image falls. In particular, a low-frequency vibration mode having a frequency lower than 100 Hz tends to cause an image failure. Therefore, in the conventional image scanning unit, in order to increase the frequency of the vibration mode, a vibration proof member such as a gel material or an insulator member has to be adopted, which causes an increase in cost.
In order to reduce the swing of the lens base member, there is also an idea to fix the intermediate section in the longitudinal direction of the carriage rails to the bottom wall of the casing. However, since there is a limit in shape accuracy of the bottom wall of the casing and the rigidity of the bottom wall is not so large, it is likely that a vibration mode that occurs in the bottom wall is transmitted to the lens base member via the carriage rails. Therefore, it is difficult to adopt the idea.